In the specialty gas and semiconductor industries there currently are no primary measurement tools to determine very low concentrations of gas phase contaminants. By "very low concentration" is meant a concentration below about 50 parts per billion. Most existing techniques rely on gas standards to form a reference which then is correlated to a physical property of an immersed material. An example of this approach is the measurement of the bulk resistance of an adsorbent material, e.g., silicalite on a ceramic substrate. Moisture in a gas standard may be adsorbed on its surface, changing the resistance of the material. Using different concentrations of the adsorbed component generates a calibration curve which then is used to determine the concentration of the component in an unknown sample. The limitation in this approach lies at concentrations under about 50 parts per billion (ppb) where component adsorption on the walls of pressurized vessels confuses calibrations and where additional errors result from cross-contamination by other components having a similar effect on the physical property being measured. A further problem is associated with the long time constants involved in reaching equilibrium at very low concentrations of components, perhaps as long as several months at the parts per trillion (ppt) level.
A more sophisticated method of measurement is a long path length spectrometer. Knowing the path length and the molar adsorptivity of a component, an estimate of its concentration can be made without resorting to standards. However, the technique lacks sensitivity because the length of the path in, for example, infrared spectrophotometry is limited to about 25 meters, resulting in a lower sensitivity limit of several ppb. The technique also suffers from long time constants needed to achieve equilibrium. In the semiconductor industry, the preferred technique is the atmospheric pressure ionization mass spectrometer (APIMS), which has high specificity and high sensitivity allowing measurements to be made in the range of 100 parts per trillion (ppt). A disadvantage of this technique is the need for a primary standard for calibration, its high price, and the requirement of high gas flow. Response times also can be very long.
The present limitations may be exemplified using an industry standard instrument, namely, the Teledyne Model 8960 which is based on coulometric hygrometry and is stated to have a sensitivity of 0.5 ppb for water vapor. However, in measuring some ultra-dry streams, the instrument read -6 ppb for water content. This exemplifies the problems associated with standards as well as the limitation of one (representative) industry standard technique. The foregoing experience is a vivid demonstration that measuring components at very low levels in, for example, high purity gases used in semiconductor fabrication, is technically inadequate. Improvements in the technology for which the ultra-pure gas stream is used are limited by current analytical techniques. There is no need to cite an extensive list of gaseous components whose concentrations at under about 50 ppb need to be measured but cannot be done because of the presently inadequate analytical needs. The invention described within provides a solution to these needs. Even more importantly, it will be recognized from the ensuing description that our method, and the sensor employed in the application of our method, can be used for measuring concentrations far above the 50 ppb level which we define as the upper limit of "low level concentration" although it will be equally obvious to one skilled in the art that our method is most applicable to measuring concentrations under about 10 ppb.
What we have done is to construct an extremely long sample cell for spectrophotometric measurements using an optical waveguide, or optical fiber, as the underlying component. The optical fiber is mounted more or less concentrically within a second cylindrical structure. The resulting annular space serves to contain a flowing gas stream which contains at least one component whose measurement is desired. The optical fiber is clad with material, generally an adsorbent, which serves to adsorb the component being measured thus effectively creating a layer of the component at the interface of the optical fiber and annular space. Optical radiation is propagated along the fiber, and the evanescent wave associated with the optical radiation is then absorbed by the interfacial layer of the component to a degree relating to the concentration of the component in the flowing gas stream. Since absorption of wavelengths in the evanescent wave is characteristic of the material being adsorbed and continues along the length of the optical fiber, one has in effect a long sample cell, i.e., a cell whose length is that of the optical fiber itself. Thus, selected wavelengths of the optical radiation propagated in the optical fiber will be absorbed by the gaseous component present at the interfacial layer. The particular wavelengths absorbed will be characteristic of the component, and the amount of propagated light absorbed will be directly proportional to the concentration of the component at the interface. This method is particularly applicable to infrared and near infrared spectroscopy, although in principal it is limited only by the optical wavelengths which are efficiently propagated within optical fibers.
A significant, critical, and unexpected point of departure of our invention from prior art centers on the nature of the cladding, i.e., the coating on the surface of the optical fiber. Conventional wisdom states that the refractive index of the cladding must be less than the refractive index of the core in order for light to be propagated along the optical fiber. In contrast, an essential feature of our invention is that the cladding have a refractive index greater than that of the optical fiber| Consequently, one would not expect the sensors of our invention to propagate optical radiation. Furthermore, Maxwell's equations can be solved only for the case where the refractive index of the cladding is less than that of the optical fiber, which means that one can predict an evanescent wave for such a condition but one can not determine whether evanescent radiation would occur even if there were propagation of optical radiation along a fiber whose cladding had a refractive index greater than that of the fiber. In short, the present state of the art would not predict propagation of optical radiation along a fiber whose cladding had a refractive index greater than that of the fiber core, and even if radiation were propagated there would be no way to predict the presence of evanescent radiation in the cladding.
In the foregoing example, the cladding of the optical fiber was an adsorbent. However, there are cases where the component whose concentration is desired to be measured has no appreciable adsorption of the wavelengths being propagated within the optical fiber. In such cases the cladding could be a catalyst or a reactant leading to materials which would absorb wavelengths in the propagated radiation and therefore could be used as an effective analytical tool. Having stated that the cladding may differ depending upon the component to be measured as well as its optical adsorption properties, many variations of the foregoing will be apparent to those skilled in the art.